Your camera is designed to do one thing: take photos. Other bells and whistles are nice but, at the end of the day, your lens, sensor, and shutter button are all that matter. Here’s a very quick breakdown of what happens inside your DSLR when shutter button is pressed.
The next couple of diagrams show what the mirror does when the shutter button is pressed. The mirror flips upward so light can pass through the lens and directly to the sensor for the image to be captured. The longer the mirror stays up, the more light the sensor can absorb – and the brighter your image will be. Check my other Workshop on light control to learn more about shutter speed and manipulating your sensor’s access to light. For shutter speeds like 1/30 (or one-thirtieth of a second), the mirror will stay up longer, allowing for a bright photo. Shutter speeds like 1/800 (or one-eight-hundredth of a second), the mirror will shut up and down a lot faster, minimizing motion blur at the expense of light.
Another way to look at it is like this: Take a shutter speed of 1/30, for example. 1 ÷ 30 = 0.0333 seconds. That means your mirror will stay up for 0.0333 seconds before coming back down. At a shutter speed of 1/800, your mirror is retracted for only 0.00125 seconds (because 1 ÷ 800 = 0.00125). That’s not a lot of time for light to pass through your lens and onto the sensor. Images at a higher shutter speed will be less blurry but, since light is hitting the sensor for less time, images will also be significantly darker.
Photography is a lot about manipulating light. Actually, it’s all about light. The goal is to balance your shutter speed with aperture and ISO to capture the best amount of light.